1. Field of the Invention
The present invention relates to a shadow mask and more particularly, to a slot-type shadow mask used for a color Cathode-Ray Tube (CRT).
2. Description of the Prior Art
FIG. 2 schematically shows the typical configuration of a shadow-mask type color CRT.
As shown in FIG. 2, a shadow-mask type color CRT 11 has a glass valve 12. The front part of the valve 12 constitutes a face panel 13 on which an image is displayed. A phosphor screen 14 is formed on the inner surface of the panel 13 within the valve 12. The screen 14 extends along the inner surface of the panel 13. A lot of stripes of phosphor materials for red (R), green (g) and blue (B) colors are horizontally and vertically arranged in the whole screen 14. Each of the stripes is usually of a vertically elongated shape, i.e., of a vertically extending strip.
A shadow mask 15 is fixed apart from the face panel 13 and is opposed to the phosphor screen 14 in the valve 12. The mask 15 has a large number of slots allowing selectively electron beams 17 to arrive at the phosphor screen 14 through the mask 15.
In the neck of the valve 12, three electron guns 16 for generating and emitting the electron beams 17 for R, G and B colors are loaded to be horizontally arranged in line. In other words, the guns 16 have the in-line structure.
A deflection yoke 18 is provided around the valve 12 in the vicinity of the neck of the valve 12, which deflects horizontally and vertically the electron beams 17, respectively.
The electron beams 17 for R, G and B colors, which are emitted from the corresponding electron guns 16, are deflected by the horizontal and vertical deflecting magnetic fields generated by the deflection yoke 18, and are scanned in the horizontal and vertical directions X and Y of the face panel 13 over the whole phosphor screen 14. The beams 17 passing through the slots of the shadow mask 15 strike the corresponding stripes of the phosphor materials on the screen 14 and excite them, thereby displaying a color image on the face panel 13.
To enhance the basic performance (i.e., contrast, brightness, and so on) of the CRT or imaging device, a black matrix film (not shown) is located on the inner surface of the face panel 13. The black matrix film is formed to be integrated with the phosphor screen 14. Specifically, the phosphor pixels for the R, G, and B colors are regularly arranged in a plane along the face panel 13. A black material such as graphite is coated to cover the remaining, exposed areas from the phosphor pixels in the same plane.
To reflect the irradiated light from the phosphor pixels effectively, a metal back film (not shown) is further located in the valve 12 apart from the screen 14 to extend along the inner surface of the screen 14. The metal back film is opposed to the shadow mask 15.
The conventional shadow mask 15 shown in FIG. 2 typically has a configuration as shown in FIG. 1.
As shown in FIG. 1, the shadow mask 15 has a plurality of slots 23 allowing selectively the electron beams 17 emitted from the electron guns 16 to arrive at the phosphor screen 14 through the mask 15. The slots 23, all of which have the same roughly rectangular shape, are regularly arranged in a rectangular slot area to form a mosaic in the horizontal and vertical directions X and Y on the face panel 13.
Between any vertically adjacent slots 23, bridge areas 24 are formed to maintain the mechanical strength of the shadow mask 15, respectively. The bridge area serve as electron-beam stopping areas.
The slots 23 are vertically elongated along the Y direction. Each of the slots 23 has the same length H. Each of the slots 23 has the same width L.sub.S, except the slots 23 located at the edges of the slot area. The width L.sub.S may be changed regularly.
The slots 23 are arranged horizontally at a specific horizontal pitch P.sub.H and vertically at a specific vertical pitch P.sub.V in the rectangular slot area. The slots 23 constitute slot columns extending along the vertical direction Y. Horizontally adjacent two ones of the slots 23 are vertically shifted at a specific value (for example, a half of the length H of the slot 23).
The slots 23 are formed in the following way.
A patterned photoresist film is formed on the surface of the metal plate on the phosphor screen side, and then, the metal plate is selectively etched by using the photoresist film. Another patterned photoresist film is formed on the opposite surface of the metal plate to the phosphor screen, and then, the metal plate is selectively etched by using the photoresist film, thereby forming penetrating holes in the plate. The penetrating holes serve as the slots 23, respectively.
The phosphor screen 14 is formed on the inner surface of the face panel 13 in the following processes.
A photoresist film is formed on the inner surface of the face panel 13 and then, the shadow mask 15 is fixed to the valve 12 to be opposite to the face panel 13. Next, the inner surface of the face panel 13 is exposed to light three times for R, G, B colors, and developed. Thus, the unexposed areas of the photoresist film, on which a black matrix is formed, are selectively removed, thereby forming a patterned photoresist film.
Subsequently, a black matrix film is coated on the patterned photoresist film thus formed, and then the remaining photoresist film is developed. Then, the remaining, exposed areas of the photoresist film and the black matrix film located thereon are selectively removed. Thus, the black matrix film is selectively left in the areas where the phosphor pixels for the R, G, and B colors are not formed.
Further, a specific phosphor material for R, G, or B color mixed with a photoresist material is coated on the inner surface of the face panel 14, and then, is exposed to light through the shadow mask 15. The exposed phosphor material is then developed. Thus, the phosphor pixels for R, G, or B color is formed on the inner surface of the panel 14. The coating process of a phosphor material and the developing process thereof are repeated three times for R, G, and B colors.
As a result, the phosphor pixels for R, G, and B colors and the black matrix are formed on the inner surface of the panel 14.
Although the shadow mask 15 has the slots 23, the obtained black matrix has a pattern of vertically extending stripes (not slots). This is because the length H of the slots 23 of the shadow mask 15 is larger than the width B.sub.w of the bridges 24, and consequently, the exposed regions of the black matrix film exposed to the light passed through the adjacent slots 23 in each slot column are combined together to form a single stripe.
For this reason, the exposure process for forming the phosphor screen 14 is affected by the slots 23 of the shadow mask 15.
With the conventional shadow mask shown in FIG. 1, the regions of the phosphor screen 14 corresponding to each slot column of the shadow mask 15 except for the first and last slot columns are exposed to not only the main light beams passed through the slots 23 included in the corresponding slot column but also the additional light beams passed through the slots 23 included in the adjacent two slot columns.
However, the regions of the phosphor screen 14 corresponding to the first or last slot columns of the shadow mask 15 are exposed to only the additional light beams passed through the slots 23 included in the second or next-to-last slot column, together with the main light beams passed through the slots 23 included in the corresponding slot column. Therefore, these regions tend to be deficiently exposed.
Such deficient exposure of the phosphor screen 14 will cause the unwanted black regions or stripes to be left on the phosphor screen 14. In other words, the wanted phosphor pixels or stripes are not formed on the screen 14. The formation of unwanted black regions are termed the "phosphor-stripe loss".